1. Field of the Invention
This invention relates to methods of sterilizing sealing articles used in syringes and other medical container devices by irradiating and autoclaving the same.
2. Description of Related Art
Elastomers are used in many important and critical applications in medical devices and pharmaceutical packaging. As a class of materials, their unique characteristics, such as flexibility, resilience, extendibility, and sealability, have proven particularly well suited for products such as catheters, syringe tips, drug vial articles, injection sites, tubing, gloves and hoses. Three primary synthetic thermoset elastomers typically are used in medical applications: polyisoprene rubber, silicone rubber, and butyl rubber. Of the three rubbers, butyl rubber has been the most common choice for articles due to its high cleanness and permeation resistance which enables the rubber to protect oxygen- and water-sensitive drugs.
Syringe plunger tips or pistons typically are made of a compressible, resilient material such as butyl rubber, because of the vulcanized rubber's ability to provide a seal between the plunger and interior housing of the syringe. Syringe plungers, like other equipment used in the care and treatment of patients, have to meet high performance standards, such as the ability to provide a tight seal between the plunger and the barrel of the syringe.
Many sterilization techniques are available today to sterilize medical devices to eliminate living organisms such as bacteria, yeasts, mold and viruses. Commonly used sterilization techniques used for medical devices include autoclaving, ethylene oxide (EtO) or gamma irradiation, as well as more recently introduced systems that involve low-temperature gas plasma and vapor phase sterilants.
One common sterilization technique is steam sterilization or autoclaving, which is a relatively simple process that exposes a device, for example, to saturated steam at temperatures of over 120° C. for a minimum of twenty minutes at a pressure of about 120 kPa (Booth, A. F. “Sterilization of Medical Devices,” Buffalo Grove, Ill.: Interpharm Press, 1999). The process is usually carried out in a pressure vessel designed to withstand the elevated temperature and pressure to kill microorganisms by destroying metabolic and structural components essential to their replication. Autoclaving is the method of choice for sterilization of heat-resistant surgical equipment and intravenous fluid as it is an efficient, reliable, rapid, relatively simple process that does not result in toxic residues. However, autoclaving can lead to hydrolysis, softening or degradation of many biomedical polymers leading to unacceptable changes in mechanical properties (Anderson, J. M. et al., “Implants and Devices,” Biomaterials Science, eds. Ratner, B. D. et al., Academic Press: London, pp. 415-420).
Radiation sterilization in the form of ionizing radiation commonly is used in hospitals for medical devices such as catheters, surgical items and critical care tools. Gamma irradiation is the most popular form of radiation sterilization and typically is used when materials are sensitive to the high temperature of autoclaving but are compatible with ionizing radiation. The bactericidal effect of gamma irradiation exerts its microbicidal effect by oxidinating biological tissue, and thus provides a simple, rapid and efficacious method of sterilization. Gamma rays are used either from a cobalt-60 (60Co) isotope source or from a machine-generated accelerated electron source. Sufficient exposures are achieved when the materials to be sterilized are moved around an exposed 60Co source for a defined period of time. The most commonly used validated dose for sterilizing medical devices is 25 kGy (Booth, A. F. “Sterilization of Medical Devices,” Buffalo Grove, Ill.: Interpharm Press,
1999). The use of gamma irradiation presents several disadvantages, however, including high capital costs and physical changes in some biomaterials, such as embrittlement, discoloration, odor generation, stiffening, softening, an increase or decrease in melt temperature and decreases in molecular weight.
Hence, despite the availability of a wide range of sterilization techniques, no single sterilization process is capable of sterilizing all medical devices without adverse effects which are caused mainly by incompatibilities between the material used in the medical device and sterilization process parameters. Syringe plunger tips made of pure butyl rubber can be autoclaved without adverse effects but they are not radiation stable, especially under stress conditions, resulting in the seal between the plunger tip and the interior housing of the syringe to be compromised causing leakage of the syringe contents.
There exists a need, therefore, to provide sterilization methods for medical devices such as syringe plunger tips using compositions which can withstand the process of sterilization and maintain high performance standards.